CD40 AND B-CELL ANTIGEN RECEPTOR DUAL TRIGGERING OF RESTING B-LYMPHOCYTES TURNS ON A PARTIAL GERMINAL CENTER PHENOTYPE

Phenotypic alterations occur when resting human B lymphocytes become g erminal center (GC) cells. These include the induction of surface CD38 , CD95 (FAS/APO-1), and carboxy-peptidase-M (CPM), a recently describe d GC marker. However, the factors that govern the in vivo induction of these surface molecules on B cells remain unknown. Here, we purified resting (CD38(-)) human B lymphocytes from tonsils in an attempt to es tablish culture conditions resulting in the induction of these three G C markers. We show that interferon (IFN) cc or IFN-gamma, as well as a ntibodies against the B cell antigen receptor (BCR), could induce CD38 on resting B lymphocytes, a phenomenon further enhanced by CD40 stimu lation. Concomitantly, CD95 was upregulated by CD40 ligation and, to a lesser extent, by IFN-gamma. By contrast, CPM expression could be upr egulated only through BCR triggering. This CPM induction was specifica lly enhanced by CD19 or CD40 ligation. CD40 + BCR stimulation of resti ng B cells with CD40 ligand-transfected fibroblastic cells in the pres ence of cross-linked anti-BCR monoclonal antibodies resulted in the co expression of CD38, CD95, and CPM. As GC cells, these cells also expre ssed CD71, CD80 (B7.1), and CD86 (B7.2), but not CD24. However, CD10() or CD44(-) B cells could not be detected in these culture conditions , suggesting that yet other signals are required for the induction of these GC markers. Consistent with a GC phenotype, CD40 + BCR-stimulate d cells exhibited reduced viability when cultured for 20 h in the abse nce of stimulus. These results first demonstrate that cotriggering of resting B cells through BCR and CD40 induces both phenotypic and funct ional GC features. They also show that IFN and CD19 triggering of rest ing B cells specifically modulate the expression of GC markers.